For years, Zofran was the most popular morning-sickness medication in the U.S. Now it’s being accused of causing birth defects. The larger issue is a drug-safety system that excludes women from clinical trials, potentially putting them and their babies at risk.

But plaintiffs’ lawyers argue that drug makers have a heightened responsibility to assure that medications likely to be used in pregnancy are safe — especially if they are used to treat a condition as common as morning sickness, and if they are marketed off-label.

“When you throw a stone in the water, you have to expect there will be ripples,” said Tobias Millrood, a Philadelphia lawyer who is one of the lead counsels. “It’s really quite that simple.”